It has been known that there are only a few basic mechanisms systems or methods for creating heat in a metallic part. Convection heating can be used which may include direct flame, immersion, radiation, electrical resistance where the heating of the metal is caused by the flow of the electricity and heat may be created by mechanical stresses or friction. Included among these has been induction heating where the heating is caused by use of magnetic fields. As is well known in the induction heating art, a metal workpiece is placed in a coil supplied with alternating current and the workpiece and the coil are linked by a magnetic field so that an induced current is present in the metal. This induced current heats the metal because of resistive losses similar to any electrical resistance heating. The coil normally becomes heated and must be cooled in order to make the heating of the workpiece as effective as possible. The density of the induced current is greatest at the surface of the workpiece and reduces as the distance from the surface increases. This phenomenon is known as the skin effect and is important because it is only within this depth that the majority of the total energy is induced and is available for heating. Typical maximum skin depths are three to four inches (8–10 cm) for low frequency applications. In all induction heating applications, the heating begins at the surface due to the eddy currents and conduction carries heat into the body of the workpiece.
Another method of heating metal parts using magnetic fields is called transfer flux heating. This method is commonly used in heating relatively thin strips of metal and transfers flux heat by a rearrangement of the induction coils so that the magnetic flux passes through the workpiece at right angles to the workpiece rather than around the workpiece as in normal induction heating. Magnetic flux passing through the workpiece induces flux lines to circulate in the plane of the strip and this results in the same eddy current loss and heating of the workpiece.
In U.S. Pat. No. 5,025,124 is disclosed an electromagnetic device for heating metal elements where the heating is accomplished by utilizing a magnetic loop for creating a high density alternating magnetic field in a metal part to be heated. The U.S. patent is based on the knowledge of replacing, in a magnetic loop, a part of the magnetic core by the metal part to be heated. In this known method the metal part is placed between the magnetic poles and may not be used in applications where it is desired to heat the metal parts from one side.
A low-frequency electromagnetic induction heater is disclosed in U.S. Pat. No. 5,053,593 that includes a plurality of leg iron cores, a corresponding yoke iron core arranged on the leg iron cores, an induction coil wound around each leg iron core, and an iron plate or metal vessel for producing heat with which the leg iron cores or induction coils are substantially in direct contact. The primary object with this induction heater is that it produces less noise than other such heaters.
In U.S. Pat. No. 3,980,858 an exciter for induction heating apparatus is disclosed provided with a number of magnetic poles and excitation windings wound on the magnetic poles. The used excitation currents are applied by a phase difference between the current applied to a central magnetic pole and current applied to peripheral magnetic poles.
The general objects of the present invention are to achieve a frying hob arrangement that enables a more accurate control of the heating, more even heating and also a more power efficient arrangement.